ABSTRACT HTLV-1 is a complex human delta retrovirus that infects 10-20 million people worldwide. While HTLV-1 infection is generally asymptomatic, 3-5% of infected individuals develop a highly malignant T cell neoplasm known as adult T cell leukemia/lymphoma (ATL) decades after infection. How HTLV-1 infection progresses to ATL is not well understood. Two viral regulatory proteins, Tax and HBZ, encoded by the sense and antisense viral transcripts respectively are thought to play indispensable roles in the oncogenic process. Tax potently activates viral transcription and I-?B kinase (IKK)/NF-?B . The latter activity is thought to drive T-cell transformation and ATL development. Paradoxically, we have found that hyperactivation of NF-?B by Tax induces a rapid cellular senescence response (Tax-IRS) mediated by two cyclin?dependent kinase inhibitors, p21CIP1/WAF1 and p27KIP1. HTLV-1 infection in culture also leads mostly to senescence. Tax-IRS, we think, constitutes a safeguard against potentially oncogenic NF-?B dysregulation. Our recent findings support the notion that escape from Tax-IRS is critical for the clonal expansion of HTLV-1-infected T cells, which invariably precedes and is requisite for ATL emergence. Importantly, a recent whole-genome analysis of ATL has revealed recurrent activating (ATLA) point mutations in key mediators of the T cell receptor and CD28 co- stimulatory signaling pathways. We hypothesize that many of the ATLA mutations are cancer-initiating and occur first in uninfected T cells where they down-regulate p21CIP1/WAF1 and p27KIP1 via IKK/NF-?B-unrelated pathways, thus preventing Tax-IRS to allow HTLV-1-infected T cells to proliferate. They likely also act in concert with HTLV-1 to immortalize/transform T cells. In agreement with the notion that ATL cells are descended from T cells harboring Tax-IRS-preventing somatic mutations, re-introduction and re-expression of tax in ATL cell lines no longer trigger a senescence response. ATL cells are characterized by the surface expression of CD4 and CD25. Many ATL cells also express Foxp3, the master transcription factor that specifies the lineage and development of the regulatory T cell Foxp3 potently represses the activities of many cellular transcription factors including NF-?B, NFAT, Runx1, ROR?T, and CREB involved in T cell activation. As the transcriptional activity of NF-?B drives senescence induction, we tested the possibility that Foxp3 may mitigate Tax-IRS to favor clonal expansion of infected T cells. Indeed, our preliminary results indicate that ectopic expression of Foxp3 does facilitate the subset. proliferation of HTLV-1-infected T cells. Here we propose to investigate the underlying causes for the clonal expansion of HTLV-1-infected T cells in two specific aims: (1) To determine whether ATL-specific activating (ATLA) mutations facilitate clonal expansion and transformation of HTLV-1-infected T cells. (2) To determine the role of Foxp3 in the proliferative expansion of HTLV-1-infected T cells.